The proposed investigation is concerned with the correlation of alterations in cell structure and function with age-associated mortality rates of the houseflies. The major goal is to identify cellular aging changes which are specifically associated with the physiological rather than the chronological rate of aging. Distinction between the physiological and the chronological age of the animals will be made by experimental manipulations of the life spans. The effects of experimentally altered life spans on metabolic rate (oxygen consumption), activities of p-phenylenediamine-mediated peroxidase, catalase, and Beta-glucoronidase, concentration of malonaldehyde, lipofuscin content, secretory activities of Malpighin tubules and fine structure of postmitotic cells in the housefly will be studied. The potential usefulness of this study lies in the identification of physiologically significant cellular alterations accompanying aging.